Optical support structures in imaging systems must possess excellent stability during testing and operations. Two of the most important structural parameters affecting stability are stiffness and Coefficient of Thermal Expansion (CTE).
The stiffness of a structural member is related to the modulus of elasticity, or modulus, for the structural material. The weight of the structural material is related to the material density. The specific modulus of a structural material is the modulus divided by the density. The specific modulus for a structural material is proportional to the stiffness to weight ratio. The specific modulus is more convenient to compute and is therefore used as a comparison parameter for structural materials.
Carbon fiber reinforced plastic (CFRP), often referred to as composite laminate materials, is a well known class of materials that is used in structural applications ranging from aircraft to fishing poles due to its high stiffness to weight ratio and high strength to weight ratio, which can be much higher than metals. The material consists of various layers, or plies, oriented and stacked in a prescribed pattern (analogous to plywood) tailored to meet the structural requirements of the member. Each layer of the laminated sheet consists of reinforcing fibers such as carbon, glass, or boron imbedded in a plastic resin. It has been demonstrated that it is possible to achieve laminates possessing near zero CTE in one or both orthogonal directions (both 1D or 2D) with high stiffness and low weight. The near-zero CTE attainable is equal to or lower (better) than that of Invar, a steel alloy with the lowest CTE possessed by a traditional metal material (0.54 .mu.m/m/.degree.C.), although, the stiffness to weight ratio of Invar is well below that which can be achieved with carbon fiber composite laminates.
The CTE of a laminate, as well as its strength and stiffness, is determined by the type of fiber selected, the orientation of each layer of those fibers, and the fiber volume fraction (ratio of the volume of fibers to the total volume of the laminate). Carbon fibers have a negative CTE in the direction of the fiber. In general, the CTE varies with the modulus of the fiber, i.e. the stiffer the fiber, the more negative the CTE. Resins have a positive CTE. For a 1D laminate and a given fiber, resin, and fiber volume fraction, it is necessary to orient some or all of the carbon fibers at some .+-.Theta angle relative to a reference direction in order to produce a laminate with a near-zero CTE in that direction. As noted herein, "reference direction" refers to the direction relative to which the fiber orientation angles are measured. However, as the Theta angle is increased, the modulus of the laminate decreases. Currently carbon fibers in the 210-345 GPa modulus range produce the highest stiffness to weight ratio laminates with near-zero CTE. Higher modulus carbon fibers (520-830 GPa) can also be used to achieve near-zero CTE laminates. Again, these fibers generally have a larger negative CTE than the 210-345 GPa modulus fibers. Consequently, larger ply angles (.+-.Theta) are required to achieve near-zero CTE. The resulting laminate modulus and therefore, specific modulus, is equal to or less than the 210-345 GPa modulus carbon fibers. The high negative CTE of the high modulus fibers prohibits increasing the laminate specific modulus above that attained by the 210-345 GPa fibers.
To achieve a near-zero CTE laminate in 2 orthogonal directions (2D laminate) a quasi-isotropic or near quasi-isotropic layup is typically used. The quasi-isotropic layup produces material properties (modulus, CTE, etc.) that are equal in the reference direction and 90.degree. to the reference direction. For a given fiber and matrix material, the fiber volume of the carbon plies in the quasi-isotropic layup is varied until a near-zero CTE laminate is achieved. Currently a 520 GPa carbon fiber is used in a quasi-isotropic layup to produce a laminate with near-zero CTE in 2 orthogonal directions (2D near-zero CTE laminate). These fibers currently produce the highest specific modulus 2D near-zero CTE laminates. Higher modulus fibers (above 520 GPa to 830 GPa) can be used, however a lower fiber volume is required. This reduces the laminate modulus to near the modulus for the 520 GPa fiber. The specific modulus of a quasi-isotropic laminate using higher modulus fibers is near the specific modulus for a quasi-isotropic laminate using the 520 GPa fiber. The higher modulus fibers are generally more costly than the 520 GPa fibers therefore the 520 GPa fibers are typically used for 2D near-zero laminates.
The current state of the art carbon laminates can be utilized to produce structural components, for example lens mounting structures in optical systems with the CTE equivalent to or lower than Invar, at a much higher specific modulus. There exists a need for structural components with low CTE's with even higher specific modulus. This need to increase specific modulus is driven by the demand to decrease structural weight, increase overall structural stiffness, reduce material cost, or a combination of these factors, while maintaining dimensional stability. This invention addresses the need for structural components exhibiting near-zero CTE, with higher stiffness to weight ratios than currently have been achieved.